Foamed products having a large number of cells formed in resins are superior in terms of properties such as lightness in weight, heat insulation, sound absorption, and stiffness at the same mass, and therefore, are used in various fields. Recently, weight reduction has particularly been regarded as being of importance because weight reduction is directly related to cost reduction in raw materials and transportation. Consequently, the number of fields to which foamed products have been applied has been further increased. Foam molding methods for resins are classified into physical foaming and chemical foaming methods according to the types of foaming agents mixed with the resins. The physical foaming methods use a physical foaming agent; an inert gas such as nitrogen and carbon dioxide, or a volatile substance such as a hydrocarbon and a fluorocarbon. The chemical foaming methods use a chemical foaming agent; an organic foaming agent such as an azo compound and a nitroso compound, or an inorganic foaming agent such as sodium bicarbonate. In injection-foaming methods using these foaming methods for the molding of resins in the injection molding methods, foamed products including cells having a cell diameter of about 80 to 300 μm in the molded products can be produced by filling a mold cavity, by injection, with a foaming-agent-containing melted resin prepared by mixing a foaming agent and a resin, and expanding the resin.
As an example of a foam-molding method for a resin, a molding method is known for producing a foamed product including cells in the molded product by mixing a chemical foaming agent or a physical foaming agent with an olefin-based resin, melting the resulting mixture, and expanding the resin by a short shot method (low-pressure molding) (see Patent Document 1). In the method disclosed in Patent Document 1, a molding machine composed of an extruder, an accumulator, and a mold is used. A foam injection molded product including cells is produced by preparing a foaming-agent-containing melted resin by mixing a resin and a chemical foaming agent or a physical foaming agent, e.g., an inert gas such as nitrogen or a volatile substance such as a hydrocarbon or a fluorocarbon, feeding the foaming-agent-containing melted resin into the accumulator with the extruder, injecting the foaming-agent-containing melted resin fed into the accumulator into the mold, and allowing the resin to expand.
In addition, as an example of a method for producing a foamed product by using a physical foaming agent, there is disclosed a method that is performed by feeding a gas such as air or a volatile substance from an extruder hopper under increased pressure together with the feeding of a resin, and conducting the melting of the resin, and allowing the resin to contain bubbles and dispersing bubbles by using the screw extruder (see Patent Document 2). In the method disclosed in Patent Document 2, an extrusion product of a sponge-like material including closed cells is produced by using a polyethylene and supplying air under an increased pressure of about 0.69 to 0.78 MPa (7 to 8 kgf/cm2). As an example of a method for producing a foamed product using carbon dioxide, which is an inert gas, as a foaming gas in its supercritical state, there is disclosed a method for markedly increasing the cell density (the number of cells per unit) compared with that in a foamed product formed by using a known chemical foaming agent or physical foaming agent (see Patent Document 3). In the method disclosed in Patent Document 3, a molded resin product including ultrafine pores having a size of less than 1 μm, which are called as microcells, in the molded product is produced by mounting a molding machine with a system composed of a supercritical-fluid booster, a supercritical-fluid feeder, and a gas cylinder; injecting carbon dioxide in a supercritical state into a melted resin from a plasticizing cylinder of this molding machine for dissolving carbon dioxide in the melted resin; filling a mold with the melted resin containing the injected carbon dioxide by injection; and expanding the resin.    Patent Document 1: JP-B-44-6080    Patent Document 2: JP-B-43-9913    Patent Document 3: JP-K-06-506724
However, the above-mentioned methods disclosed in Patent Documents 1 to 3 have the following problems. That is, in the method disclosed in Patent Document 1, when a resin and an organic chemical foaming agent such as an azo compound or a nitroso compound are fed for molding, corrosive decomposition products such as ammonia, carbon monoxide, steam vapor, cyanic acid, and isocyanic acid are generated by heat decomposition. Problematically, these decomposition products are discharged to the atmosphere and also remain in the molded product. When a physical foaming agent that is free from the generation of decomposition products, i.e., a volatile substance such as a hydrocarbon or a fluorocarbon is fed for the molding, the exhaust of the gas to the atmosphere is regulated as a material which causes pollution and destruction of the environment. Thus, the use of such a gas is problematic. In the method disclosed in Patent Document 2, the gas cannot be finely dispersed in the foaming-agent-containing melted resin. Therefore, it is difficult to produce a foamed product having a predetermined cell density and cell diameter; which is problematic. Furthermore, in case of the method disclosed in Patent Document 3, since the installation of a supercritical-fluid generator and feeder are indispensable, these apparatuses utilize a high-pressure gas and therefore are under legal restrictions, and thus, there are problems that the introduction of facilities and the handling of them become troublesome.
The present invention has been accomplished in view of the above-mentioned problems of the known technologies and aims to provide an injection-foaming method for a thermoplastic resin which can simply, safely, and efficiently produce a foamed product having a predetermined cell diameter and density. Furthermore, the present invention also aims to provide an injection-foaming method for a thermoplastic resin which is environmentally friendly.